1. Field of the Invention
The present invention relates to a boring tool such as a drill and, more particularly, to an improvement in a cutting tool suitable for use in the boring of aluminum castings.
2. Description of the Related Art and Problems
A known drill reamer, as an example of the boring tool of the type mentioned above, is shown in FIGS. 4 to 8. This drill reamer has tool body 1 which has a rectangular cross-section taken along the axis thereof. A pair of chip discharge flutes 2, 2, each having a V-shaped cross-section, are formed in the side surface of the tool body 1 so as to extend linearly from the front end to the rear end of the tool body. The edge line where the wall surface of each flute 2 facing forwardly in the rotation direction intersects the end relief surface forms an end cutting edge 3, while the edge line where the above-mentioned wall surface intersects an outer peripheral margin forms an outer peripheral cutting edge 5. In this drill reamer, the drill end portion on the trailing side of the outer peripheral margin 4 is cut to provide a flat land portion 6 and a second margin 7 is formed on the trailing side of the land portion 6.
This drill reamer exhibits a high degree of rigidity by virtue of the fact that the chip discharge flutes 2, 2 are straight. In addition, since the drill reamer is guided by four margins, vibration is effectively suppressed during drilling, thus offering a high level of machining precision.
FIGS. 9 and 10 show a different known drill reamer of the type which is disclosed in Japanese Utility Model Laid-Open No. 57-100419. This drill reamer has a pair of first chip discharge flutes 11 formed in the outer peripheral surface. A second chip discharge flute 13 and a third chip discharge flute 14 are arranged in the mentioned order in the direction of rotation in each of lands 12 defined between the pair of first chip discharge flutes 11. A rough cutting edge 11a, a medium-finish cutting edge 13a, and a finish cutting edge 14a are formed on the edge lines where the end relief surface meets the forwardly facing wall surfaces of the first, second, and third chip discharge flutes 11, 13 and 14, respectively. These cutting edges have different diameters which increase in a stepped manner.
It is said that in this type of drill reamer, the boring load is shared by the plural cutting edges so that the burden on each cutting edge, i.e., the amount of the metal to be removed during cutting by each cutting edge, is reduced, which effectively improves precision of the boring, as well as improving the surface roughness, while extending the life of the cutting edge.
However, these known drill reamers suffer from the following disadvantages. Namely, the drill reamer of the first-mentioned type is liable to generate much heat due to friction between the outer peripheral margins and the bore wall, because the number of the outer peripheral margins is as large as four. In addition, the chips of the cut metal tend to be introduced to the second margin because the lands are flattened. Consequently, the cut metal chips tend to be caught between the second margin and the bore wall so as to be welded therebetween, thus roughening the finished surface and increasing the power required for the drilling.
In the drill reamer of the second-mentioned type, the total cutting resistance is greater than that produced by two-edged drills because it has many, i.e., six, cutting edges. On the other hand, only two margins are available for guiding the tool body during the cutting. Consequently, the tool body is liable to vibrate. In addition, only a pair of cutting edges, more specifically the finish cutting edges 14a, 14a, take part in the finishing of the bore, from among six cutting edges on the drilling reamer. Consequently, there is a practical limit in the improvement in the boring precision and in the fineness of the finished surface.